It is well recognized by persons skilled in the art of oil recovery that there are formations which contain petroleum whose viscosity is so great that little or no primary production is possible. Some form of supplemental oil recovery or enhanced oil recovery must be applied to these formations in order to decrease the viscosity of the petroleum to a level so it will flow or can be displaced through the formation to the production wells and from there recovered to the surface of the earth. Thermal recovery processes have been used successfully for recovering viscous oil from such formations, and steam flooding is the most successful thermal oil recovery method employed commercially. Steam may be utilized for thermal stimulation of viscous oil formations in what is referred to as a "huff and puff" technique in which steam is injected into a well, allowed to remain in the formation for a short period, after which oil is recovered from the formation by means of the same well as was used for steam injection. A somewhat more successful technique employs steam in a steam drive or steam throughput process in which steam is injected into the formation on a more or less continuous basis by means of an injection well and oil is recovered from the formation by a spaced apart production well. The technique is somewhat more effective in many applications than the single well steam stimulation process since it both reduces the viscosity of petroleum and displaces petroleum through the formation, thus encouraging oil production from a remotely located production well.
While this process is effective with respect to the portion of the formation through which the steam travels, poor vertical conformance is often experienced in steam drive oil recovery processes. A major cause of poor vertical conformance is that steam is less dense than other fluids present in the earth formation, and so steam migrates to the upper portion of the permeable formation and channels across the top of the oil formation to the remotely located production well. This is referred to in the art as steam override. Once steam override has occurred in the upper portion of a formation, the permeability of the steam swept zone is increased due to the desaturation or removal of petroleum from the portion of the formation through which steam has channeled. Thus, subsequently injected steam will migrate almost exclusively through the steam-swept channel and very little of the injected steam will move into the lower portion of the formation, and thus very little additional viscous petroleum will be recovered from the lower portion of the formation. While steam drive processes effectively reduce the oil saturation of the portion of the formation through which steam passes by a significant amount, the portion of the recovery zone between the injection and production system actually contacted by steam is often less than 50% of the total volume of that recovery zone, and so a significant amount of viscous petroleum remains in the formation after completion of the steam drive oil recovery process. The severity of the poor vertical conformance problem increases with the thickness of the oil formation, vertical permeability and with the viscosity of petroleum contained in the earth formation.
Steam drive oil recovery processes may also be used in more conventional, low viscosity oil-containing formation, and steam override is also encountered in these cases.
Since the viscosity of steam is much less than the viscosity of petroleum, poor horizontal conformance is also encountered in steam throughput processes. This further reduces the percentage of the total volume within the pattern of wells employed in steam drive processes actually swept by injected steam.
In view of the foregoing discussion, and the large deposits of viscous petroleum from which only a small portion of the in place petroleum can be recovered because of the horizontal and vertical conformance problems, it can be appreciated that there is a serious need for an improved steam drive oil recovery method suitable for use in recovering viscous petroleum from relatively thick formations, which results in improved vertical conformance.